Accpetance Tests are executable specifications. They verify a product’s correctness from a business perspective.
According to Andy Dassing (http://tech.groups.yahoo.com/group/fitnesse/message/10115)
Acceptance Tests insure the right code is built … while Unit Tests insure the code is build right.
Acceptance Tests are written in the language of the domain.
An Acceptance Test should verify a single aspect of a User Story.
When all Acceptance Tests pass, the system is complete.
Mentioned in Dispatches 